bioRxiv preprint doi: https://doi.org/10.1101/318121; this version posted September 21, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. Open Access RESEARCH ARTICLE Palaeobiological INFERENCES BASED ON LONG BONE EPIPHYSEAL AND DIAPHYSEAL STRUCTURE - THE FORELIMB OF XENARTHRANS (Mammalia) Eli Amson & John A NyakaturA Cite as: Amson E AND NyakaturA JA. (2018). Palaeobiological INFERENCES BASED ON LONG BONE EPIPHYSEAL AND DIAPHYSEAL STRUCTURE - THE FORELIMB OF XENARTHRANS (Mammalia). bioRxiv, 318121, ver. 5 peer-rEVIEWED AND RECOMMENDED BY PCI Paleo. DOI: 10.1101/318121 Peer-rEVIEWED AND RECOMMENDED BY Peer Community IN Paleontology Recommendation DOI: 10.24072/pci.paleo.100001 Published: 21 September 2018 Recommended By: AleXANDRA Houssaye Based ON REVIEWS By: AndrEW Pitsillides AND AN ANONYMOUS REVIEWER PEER COMMUNITY IN PALEONTOLOGY bioRxiv preprint doi: https://doi.org/10.1101/318121; this version posted September 21, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. Preprint uploaded to Biorxiv on 21 September 2018 Version 5, © 2018 Amson, Nyakatura CC-BY-NC-ND 4.0 Palaeobiological inferences based on long bone epiphyseal and diaphyseal structure - the forelimb of xenarthrans (Mammalia) Eli Amson1,2, John A Nyakatura2 1Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany; [email protected] 2AG Morphologie und Formengeschichte, Institut für Biologie & Bild Wissen Gestaltung. Ein interdisziplinäres Labor, Humboldt-Universität, Berlin, Germany; [email protected] ORCID 0000-0003-1474-9613 (Eli Amson) 0000-0001-8088-8684 (John A Nyakatura) This article has been peer-reviewed and recommended by Peer Community in Paleontology (https://dx.doi.org/10.24072/pci.paleo.100001). ABSTRACT Trabecular architecture (i.e., the main orientation of the bone trabeculae, their number, mean thickness, spacing, etc.) has been shown experimentally to adapt with great accuracy and sensitivity to the loadings applied to the bone during life. However, the potential of trabecular parameters used as a proxy for the mechanical environment of an organism’s organ to help reconstruct the lifestyle of extinct taxa has only recently started to be exploited. Furthermore, these parameters are rarely combined to the long-used mid-diaphyseal parameters to inform such reconstructions. Here we investigate xenarthrans, for which functional and ecological reconstructions of extinct forms are particularly important in order to improve our macroevolutionary understanding of their main constitutive clades, i.e., the Tardigrada (sloths), Vermilingua (anteaters), and Cingulata (armadillos and extinct close relatives). The lifestyles of modern xenarthrans can be classified as fully terrestrial and highly fossorial (armadillos), arboreal (partly to fully) and hook-and-pull digging (anteaters), or suspensory (fully arboreal) and non-fossorial (sloths). The degree of arboreality and fossoriality of some extinct forms, “ground sloths” in particular, is highly debated. We used high-resolution computed tomography to compare the epiphyseal 3D architecture and mid-diaphyseal structure of the forelimb bones of extant and extinct xenarthrans. The comparative approach employed aims at inferring the most probable lifestyle of extinct taxa, using phylogenetically informed discriminant analyses. Several challenges preventing the attribution of one of the extant xenarthran lifestyles to the sampled extinct sloths were identified. Differing from that of the larger “ground sloths”, the bone structure of the small-sized Hapalops (Miocene of Argentina), however, was found as significantly more similar to that of extant sloths, even when accounting for the phylogenetic signal. Keywords: Bone structure; Forelimb; Locomotion; Palaeobiological inferences; Trabeculae; Xenarthra (Ruff et al. 2006). This was argued for trabecular INTRODUCTION bone, which reacts to loading with great accuracy and sensitivity (Barak et al. 2011). This was also argued Bone structure is intensively studied in analyses for cortical bone, even though the latter is expected concerned with functional anatomy because it is to be less plastic, at least in part due to its lower argued to be extremely plastic. While a genetic remodeling rate (see review of Kivell, 2016). blueprint influences bone structure, it has been Comparative studies focusing on either trabeculae or shown to adapt during life (and especially at an early cortical structure intend to leverage this great ontogenetic stage) to its mechanical environment plasticity to associate structural phenotypes to 1 bioRxiv preprint doi: https://doi.org/10.1101/318121; this version posted September 21, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. PREPRINT lifestyles or functional uses of a limb. This has been however, that Tsegai et al. (2017), also used this achieved in some analyses (as recently exemplified holistic analysis and performed a Principal by Georgiou et al. 2018; Ryan et al. 2018; Tsegai et Component Analysis (even though in that case the al. 2018) but not all of them (see review of Kivell focus was on trabecular bone architecture and 2016), suggesting that some confounding factors are cortical bone thickness at the articular surface). likely to be at play, and more generally that the Skinner et al. (2015) and Stephens et al. (2016) also approach is limited. For trabecular bone in particular, used Gross et al. (2014)’s method, but focused on important intraspecific variation has been trabecular architecture only. This approach is documented (e.g., in Pongo; Tsegai et al. 2013; particularly relevant for medium- to large-sized Georgiou et al. 2018). Nevertheless, the fact that mammals such as Pan or Homo, for which the some analyses successfully distinguished ecological epiphyses include a complex trabecular architecture groups might indicate that broad differences of bone with distinct zones of different arrangement (such as structure among lifestyles can exceed, at least in the so-called vertical and horizontal trabecular some cases, individual variability. Because fossil columns in the femoral neck; Hammer 2010). One bone cross-sections at mid-diaphysis have been can note that an entirely different approach, not produced for over a century and a half (Kolb et al. relying on the measurement of these parameters, but 2015), a large number of mid-diaphyseal data related on micro-finite element analysis, was also applied to to extinct taxa have been acquired, and successfully a primate (Huynh Nguyen et al. 2014). To our exploited for palaeobiological inferences (e.g., knowledge, epiphyseal trabecular and mid- Germain & Laurin, 2005). Fossil three-dimensional diaphyseal parameters have never been combined in (3D) trabecular architecture has been much less a functional analysis about non-primate taxa, and no investigated, as, to our knowledge, only few studies analysis used both trabecular and cross-sectional have been published, which are all focussing on parameters in the same discriminant test. primates (DeSilva & Devlin 2012; Barak et al. 2013; References to bone structure in “ground sloths”, Su et al. 2013; Skinner et al. 2015; Su & Carlson Megatherium in particular, date back to the 19th 2017; Ryan et al. 2018). century (Owen 1861). But it is only fairly recently that In general terms, it is assumed that the diaphysis quantification of bone structure was performed of long bones tends to be exposed to mostly bending (Straehl et al. 2013; see review of Amson & and torsion, and to a lesser extent axial compression Nyakatura 2017). Straehl et al. (2013) examined (Carter & Beaupré 2001). On the other hand, the compactness profile of a mid-diaphyseal section in architecture of epiphyseal trabeculae is usually the limb long bones of various extant and extinct related to compressive and tensile strains (Biewener xenarthrans. They found that most armadillos were et al. 1996; Pontzer et al. 2006; Barak et al. 2011). characterized by a humeral mid-diaphysis that is Trabecular and cortical compartments are hence relatively more compact than that of the femur. expected to have distinct mechanical properties, Subsequently, Amson et al. (2017a) studied the which do not necessarily co-vary. To combine them epiphyseal trabecular architecture in extant in a single analysis, it can therefore be argued that xenarthrans, and found that some parameters, the the structural parameters deriving from these two degree of anisotropy (DA) in particular, differed types of structures should be considered as distinct among functional categories. (univariate) variables. Because trabecular and Indeed, xenarthrans are marked by distinct cortical structures have independently yielded a lifestyles that can be used to define functional functional signal, such a combined analysis could categories. Extant xenarthrans were categorized by potentially help in our endeavours to associate a bone Amson et al. (2017a) as fully arboreal and non- overall